The world's oldest known photograph has been sold to the French National Library for 450,000 euros ($392,000).

The image of an engraving depicting a man leading a horse was made in 1825 by Nicephore Niepce, who invented a technique known as heliogravure.

It might not first be recognised as what we call a photograph

Sothebys expert Philippe Garner

The French Government declared the work - the date of which is confirmed in a letter from Niepce - to be a national treasure meaning it must stay in the country.

Our correspondent says this kept the price down and allowed the library to make the acquisition.

By coating a copper plate with a kind of light-sensitive bitumen, Niepce found he could take a negative imprint of an image and use that to create a new picture - the first photograph.

He used the method to make a copy of the Flemish 17th Century print, and a letter to his son Isidore shows it is the earliest surviving testament to his breakthrough.

Action of light

Philippe Garner, who was in charge of the sale for the auctioneers Sothebys, in Paris said: "If you look at this, it might not first be recognised as what we call today a photograph.

"And in fact, it is printed ink on paper.

"The crucial stage in creating this plate, however, is the etching by the action of light on light-sensitive chemicals of a metal plate which is then used as the printing plate."

The photograph was part of the collection of the Paris-based booksellers Marie-Therese and Andre Jammes, who are revered in the photographic world for their lifelong commitment to archiving historic works.

Also on sale are early works by Hippolyte Fizeau and Jacques Louis Daguerre, the collection of 19th Century master Charles Negre, portraits of literary figures such as Colette by Laure Albin-Guillot and an album of 77 photographs by Robert Doisneau.